The present invention relates to a phantom object (also referred to as test object) for the quality control of devices for radiation therapy treatment, to a method for the manufacture of the phantom object and to methods of use of this phantom object, consisting in a method of checking the coincidence, the orthogonality and the spatial position of locating means of the three theoretical axes of rotation of a device for radiation therapy treatment and in a method of searching the isocenter of a device for radiation therapy treatment using the phantom object, the latter being the two aspects of quality control.
Treatment by radiation therapy for the treatment of cancer should be performed so that the rays are targeted on the tumor and avoid to the maximum extent healthy tissue around the treated tumor.
To ensure the targeting of the rays on the tumor, several quality control tests of the treatment devices have been developed, which are performed on the treatment device before the radiation therapy treatment of a patient.
A conventional device for radiation therapy treatment, shown for example in FIG. 3, comprises a stand, bearing at one end an irradiation head which ends by a collimator which allows to delineate the radiation beam (or irradiation beam) and at the other end an imager referred to as portal imager which allows to make digital radiographies of an object positioned between the collimator and the imager, generally on a treatment table also referred to as patient support.
The treatment device comprises three axes of rotation, shown on FIG. 3: the horizontal axis of rotation of the stand, which allows the rotation of the irradiation head around the treated patient, the axis of rotation of the collimator, which is an axis passing by the center of the collimator, and perpendicular to the horizontal axis of rotation of the stand, this axis being coincident with the vertical axis passing by the center of the collimator when the rotation angle of the stand is null, and the vertical axis of rotation of the patient support, which is an axis passing by the center of the collimator, when the rotation angle of the stand is null.
The point of convergence of these three axes is called the isocenter.
The position and the “size” of this isocenter are crucial to be known because it is on this point, in the three-dimensional space of the treatment room, that the center of the tumor to be treated will be positioned, so that it can be irradiated by means of multiple concentric beams. This isocenter point is represented physically in the treatment rooms by five orthogonal laser layers, two frontal, one sagittal and two transverse. These layers will allow the alignment of three reference points (one anterior and two lateral) represented physically on the patient skin (tattoo) or on the surface of a restraining system used for positioning very precisely the patient, during the phase of preparation and planning of the treatment.
Both transverse layers are ideally in a vertical plane, orthogonal to the longitudinal direction of the patient support when its rotation angle is null, while the two frontal layers are ideally in a horizontal plane, and the sagittal layer is in a vertical plane, orthogonal to the plane of the transverse layers.
In order to ensure the geometrical precision of the irradiation, it is of primary importance to check the proper alignment of the theoretical isocenter represented physically by the intersection of the locating laser layers with the real isocenter of the treatment device, which corresponds to the intersection of the three real axes of rotation of the device for radiation therapy treatment. Any misaligning between the real isocenter and the theoretical isocenter would result, on the one hand, in an incomplete irradiation of the tumor, which can induce a resurgence of the disease, and, on the other hand, in an irradiation of the healthy tissue around the tumor, said irradiation being liable to produce severe complications.
Winston-Lutz (W&L) test allows checking the coincidence of the theoretical isocenter and of the real isocenter. It consists in aligning a radio-opaque ball, therefore of high electron density, especially of a density close to the one of a metal, spherical (mostly in steel) on the theoretical isocenter of the device (intersection of the locating laser layers), and in performing multiple radiographies of this object with the X-rays from the treatment beams. By performing such radiographies for different rotation angles of the stand, the position and the “size” of the axis of rotation of the stand is determined. By performing these radiographies for different rotation angles of the patient support, the position and the “size” of the axis of rotation of the patient support is determined. Finally, by performing these radiographies for different rotation angles of the collimator, the position and the “size” of the axis of rotation of the collimator is determined. It should be noted that, during the W&L test, a series of radiographies is performed around a single axis of rotation at a time, while the other rotation angles around the two other axes are set to 0°. The angle of 0° for the stand corresponds to the vertical position thereof (as shown on FIG. 4), the angle of 0° for the patient support corresponds to the position of the patient support in which the longitudinal direction of the patient support is aligned with the axis of rotation of the stand, and the angle of 0° for the collimator corresponds to a preset angle in the collimator. This Winston-Lutz test is described on the Web page http://www.wienkav.at/kav/kfj/91033454/physik/aS500/aS500_sphere.htm
The word “size” used to define a feature of an axis of rotation, as used above for the real axes of rotation, corresponds to the mean diameter of the considered axis of rotation, which, in the case of the real axes of rotation, does not correspond exactly to a line, but is contained in a very thin cylinder.
If a phantom object consisting of a radio-opaque ball is perfectly positioned on the real isocenter of the device, which means that the theoretical isocenter (locating means) matches perfectly with the real isocenter of the treatment device, then the image of the ball on the recurrent radiographies is constant.
Otherwise, the image of the radio-opaque ball follows a movement, the analysis of which allows to find the shifts to be produced in order to realign the theoretical isocenter on the real isocenter of the treatment device.
The radio-opaque ball should be sufficiently small-sized so that it can be contained in an irradiation beam of small section (about 50 mm) delineated by the collimator. Indeed, the movements of the radio-opaque ball are not studied with respect to an origin related to the radiation detector, but with respect to the center of the irradiation beam detected on the radiation detector. Indeed, this allows overcoming a possible movement of the radiation detector during the rotation of the rotating stand, this movement being interpreted as a defect on the isocenter of the treatment device. In order to limit also a possible movement with the rotation of the stand of the system of limitation of the beam contained in the collimator, small section beams are used. Several phantom objects exist for performing the W&L test. The most used phantom object is a radio-opaque ball with a diameter between 2 or 10 mm, which has to be aligned on the five locating laser layers. This procedure is difficult because, generally, the ball cannot bear alignment marks with the laser layers because of its small size. Even if it was engraved, still because of its small size, the alignment of the lasers can be controlled only on a reduced surface of the ball, which results in an inaccuracy in positioning the ball of about the shifts between the real and theoretical isocenters which are to be measured and corrected. A solution would consist in a further reduction of the size of the ball to make more accurate the laser alignment, but in this case, it would be difficult to locate the less and less opaque ball in the recurrent radiographies.
Other phantom objects rely on the principle consisting in enclosing the ball in a plastics parallelepiped with small sizes (20 cm3) comprising a reticule engraved on three of the six faces, the anterior face and the two lateral faces. This time, the alignment of the ball is facilitated but the parallelepiped geometry makes the recurrent radiographies disparate because of the variations of the image of the projection of the parallelepiped with the projection angle. This disadvantage prevents an optimal detection of the center of the radio-opaque ball, especially with the use of software of automatic analysis of the radiographies. Moreover, the parallelepiped geometry prevents from checking the coincidence of the laser layers and the small sizes of the encompassing parallelepiped limit the accuracy of the prior control of the orthogonality and of the spatial position (horizontality and verticality) of the laser layers.
Thus, the currently used phantom object exhibit two major disadvantages: on the one hand, the inaccuracy of the positioning of the radio-opaque ball at the intersection of the five laser layers, i.e. on the theoretical isocenter, and on the other hand, the impossibility to check, prior or complementary to the W&L test, the orthogonality, the coincidence and the spatial position of the laser layers, therefore of the three theoretical axes.
Indeed, two of the three theoretical axes (the axis of rotation of the collimator and of the patient support) must be vertical, and the last theoretical axis (axis of rotation of the stand) must be horizontal.